ABSTRACT Clinical evidence indicates that moderate to vigorous exercise is a powerful means to reduce metastatic cancer incidence. However, the mechanisms of this beneficial influence are not fully understood. Our present application is specially focused on the mechanisms of tumor cell extravasation into the brain. Such an emphasis on brain metastases is consistent with our interest in the blood-brain barrier physiology and pathology. In addition, brain metastases are one of the leadings causes of cancer-related morbidity and mortality. The central hypothesis of the present application is that exercise protects against the development of blood- borne brain metastases by increasing antioxidant capacity and modulating redox- regulated responses in the capillary endothelium. To study this hypothesis, we will employ an animal model of wheel running mice that mimics the voluntary pattern of human exercise. We will specifically focus on the exercise-mediated protection against vascular mechanisms of tumor cell extravasation via disruption of tight junction proteins of the endothelium. Tight junctions are the critical components of the brain capillaries which regulate the integrity of the blood-brain barrier. Mechanistically, the main emphasis will be placed on the involvement of the Ras and Rho signaling in alterations of phosphorylation and expression of tight junction proteins. The proposed research combines elements of exercise physiology, clinical approaches (namely, tumor dissemination and growth), cancer progression, and molecular and vascular biology. In addition, we will employ advanced systems biology approaches. Novelty and significance of the present proposal are related to our focus on the blood-brain barrier in brain metastasis, as tumor extravasation occurs at the level of the cerebrovasculature endothelium and the evaluation of the protective effects of physical activity on tumor dissemination and growth. We believe that the data obtained from this proposal will provide evidence that even moderate exercise can significantly protect against the development of blood-brain metastases. Furthermore, a better understanding of the pathophysiological regulation of BBB molecular and functional properties is critical in assessing brain metastasis etiology and in identifying future drug targets to develop more effective therapeutic approaches.